1. Field of the Invention
The present invention relates to a method of backside irradiating flexographic plates with ionizing radiation prior to curing radiation curable polyurethane elastomeric compositions and substrates and a method of making flexographic printing plates produced with those compositions. The plates are preferably developable in a solventless absorption processing method, but solvent development plates show advantage with this process. This invention describes compositions which are resistant to swelling when immersed in water or solvent based flexographic inks, which resistance property is critical to the use of these compositions in the form of flexographic printing plates.
2. Background of the Art
Flexography is a term that broadly applies to a printing format using a flexible substrate bearing an elastomeric or rubbery relief printing surface.
The first flexographic printing plates were produced from natural or synthetic rubber compositions which were cured chemically under heat and pressure in a mold utilizing conventional rubber curatives such as mercapto compounds (Flexography: Principles and Practices, 3rd Edition, Flexographic Technical Association, p. 158-162). More recently, photopolymer elastomeric compositions (elastomer containing compositions curable upon exposure to actinic radiation) useful to produce relief printing plates have been described. For example, U.S. Pat. No. 4,369,246 and 4,423,135 describe solvent-insoluble, elastomeric printing relief plates which are formed by applying to a sheet support a layer of a photosensitive composition comprising (1) at least 30 weight % of a solvent-soluble co-polymer containing at least 2 thermoplastic, non-elastomeric blocks of glass transition temperature above 25.degree. C. and average molecular weight 2000-100000 and between these blocks, an elastomeric block copolymer having a glass transition temperature below 10.degree. C. and average molecular weight 25,000-1,000,000; (2) at least 1 weight % of an addition polymerizable compound containing at least one terminal ethylenic group; and (3) a polymerization initiator at a dry thickness of 0.005-0.250 inch. A flexible polymer film and flexible cover sheet are laminated to the composition layer. The plate is formed by stripping off the cover sheet, imagewise exposing the layer to actinic radiation through the film, and removing the film and unexposed areas of the layer by solvent washing. Solvents such as perchloroethylene (1,1,1 trichloroethylene) either alone or in combination with alcohols such as n-butanol are utilized. Likewise, EP Pat. 261,910 describes a further example of a water-developable flexographic relief printing plate comprised of (1) monomers and polymers of acrylic acid esters and (2) a ketone photopolymerizing/photocrosslinking agent, which are coated on a support sheet. Following imagewise exposure (to promote crosslinking), the relief areas of the plate are produced by washout with an aqueous developer. After washout, all of the flexographic platemaking compositions and methods described heretofore require drying of the plate for extended periods (1 to 24 hours) to remove entrained developer solution and then are subjected to a post-finishing process (chemical or photochemical) to reduce surface tack of the plate before use on a printing press. In addition to the extended time periods required to produce flexographic printing plates by the aforementioned technologies (by reason of the multiplicity of processing steps required in series), these technologies also produce potentially toxic by-product wastes in the development procedure. In the case of the solvent-washout technology, organic solvent waste is generated which is potentially toxic in the form of both the solvent and the addition polymerizable compound 2) which contains at least one terminal ethylenic group. Likewise, the aqueous washout plate technologies also produce a contaminated waste water by-product stream which may contain similar addition polymerizable compounds that may have cytotoxic effects.
Many different types of monomers and cross-linkable resins are known in the polymer art, their properties can be adjusted as taught in the art to provide rigidity, flexibility, or other properties. Particularly good materials related to the compositions of the present invention are shown in U.S. Pat. Nos. 4,578,504; 4,638,040; and 4,786,657.
Various types of curable polyurethane elastomeric compositions are known. The term "elastomer" or "elastomeric" is used to refer to rubbers or polymers that have resiliency properties similar to those of rubber. In particular, the term elastomer reflects the property of the material that it can undergo a substantial elongation and then return to its original dimensions upon release of the stress elongating the elastomer. In all cases an elastomer must be able to undergo at least 10% elongation (at a thickness of 0.5 mm) and return to its original dimensions after being held at that elongation for 2 seconds and after being allowed 1 minute relaxation time. More typically an elastomer can undergo 25% elongation without exceeding its elastic limit. In some cases elastomers can undergo elongation to as much as 300% or more of its original dimensions without tearing or exceeding the elastic limit of the composition. Elastomers are typically defined to reflect this elasticity as in ASTM Designation D883-866 as a macromolecular material that at room temperature returns rapidly to approximately its initial dimensions and shape after substantial deformation by a weak stress and release of the stress. ASTM Designation D412-87 can be an appropriate procedure for testing rubber properties in tension to evaluate elastomeric properties. Generally, such compositions include relatively high molecular weight compounds which, upon curing, form an integrated network or structure. The curing may be by a variety of means, including: through the use of chemical curing agents, catalysts, and/or irradiation. The final physical properties of the cured material are a function of a variety of factors, most notably: number and weight average polymer molecular weights; the melting or softening point of the reinforcing domains (hard segment) of the elastomer (which, for example, can be determined according to ASTM Designation D1238-86); the percent by weight of the elastomer composition which comprises the hard segment domains; the structure of the toughening or soft segment (low Tg) portion of the elastomer composition; the cross-link density (average molecular weight between crosslinks); and the nature and levels of additives or adjuvants, etc. The term "cured", as used herein, means cross-linked or chemically transformed to a thermoset (non-melting) or relatively insoluble condition.